The present invention relates to a new type of foundation for a nuclear reactor confinement enclosure.
The confinement enclosures used in the nuclear industry essentially comprise a dome surmounting a cylindrical skirt bearing on a foundation. They are formed from an inner mild steel skin, which serves to ensure the necessary sealing and which is surrounded by a concrete shell ensuring the strength of the overall assembly. The stresses undergone by the enclosure are in fact numerous and large. The foundation supports the weight of the equipment and the internal structures, such as the vessel, the steam generators, the primary circuit and the pumps. The skirt and the dome are subject to the conventional forces of civil engineering structures, such as those produced by wind, and must also be able to withstand catastrophic events, such as earthquakes and pressurized circuit fractures, which have the effect of producing a significant overpressure within the enclosure. The latter event makes it necessary for the confinement enclosure to be in tension, which is not desirable in the case of concrete. It is for this reason that numerous prestressing cables are positioned along the meridians of the dome and descend along the cylindrical skirt, the ends thereof being fixed to the periphery of the foundation.
However, this prestressing procedure causes a significant problem for the foundation and it is on the basis thereof that the present invention arises. Thus, the prestressing cables, through transmitting the internal pressure in the case of an incident, produce a vertical rising force on the periphery of the foundation, while the weight of the various installations placed on said foundation causes downward forces largely applied towards the periphery thereof. Thus, there are significant shear stresses, which must be taken up with the aid of prestressed reinforced concrete. Therefore the cost of the structure is greatly increased. Moreover, the forces caused in this way can lead to bowing or warping of the foundation, which tends to lift at its periphery, and this causes a loss of cohesion of the soil for the foundations and significantly reduced stability of the confinement enclosure. In the case of confinement enclosures with a diameter of about 50 m, the warping can be sufficient to justify the establishment of a second foundation supporting the installations located in the confinement enclosure; this is located above the first foundation and is separated therefrom by a layer of deformable materials. This clearly also leads to an increase in the construction costs.
Finally, the installation and maintenance of the ends of the prestressing cables require a significant construction volume obtained by digging out a circular ring below the actual foundation. As a result, the drainage of that part of the ground or soil located within the ring is much more difficult.
An improvement to this concept was proposed in the French Pat. No. 7533441. According to the latter, the foundation is replaced by a more complex structure comprising a lower horizontal footing, a second footing located above the first and which constituting the ground of the confinement enclosure and a certain number of vertical walls defining the space between the two footings and breaking it up into zones. The heads of the prestressing cables of the skirt and dome are consequently located between the two footings. This arrangement makes it possible to eliminate the drainage problems produced by the arrangement of a lower annular ring around the foundation, while also saving a considerble amount of space. However, the problems referred to hereinbefore concerning the poor working of the concrete in tension and bending still occur.